(1) Field of the Invention
The present invention relates to the field of methods and devices for controlling the operation of rotorcraft engines. More specifically, the present invention relates to methods and devices for protecting a rotorcraft engine by causing the engine to stop if it is in an overspeed situation.
(2) Description of Related Art
Rotorcraft are rotary wing aircraft in which at least lift is provided by at least one main rotor having a substantially vertical axis. In the specific context of a helicopter, the main rotor provides the rotorcraft not only with lift, but also with propulsion to advance in any direction.
Rotorcraft are also fitted with an anti-torque device that guides them in yaw, such as at least one auxiliary rotor having an axis that is substantially horizontal. By way of example, such an auxiliary rotor is a tail rotor, or else it may be formed by a propulsive propeller in the context of a helicopter having high propulsion speed. It is possible to use other anti-torque devices, such as for example anti-torque devices that blow air.
The flight attitude of a rotorcraft can be modified by a pilot imparting collective and/or cyclic variation to the pitch of the blades making up the rotary wing of the main rotor. Collective variation of the pitch of the blades of the main rotor serves to modify the lift applied to the rotorcraft and consequently enables it to move in a vertical direction. Cyclic variation of the pitch of the blades of the main rotor serves to modify the propulsion of the rotorcraft along other propulsion directions in the specific circumstance of a helicopter.
Furthermore, variation in the collective pitch of an auxiliary rotor or making use of any other anti-torque device serves to stabilize the progress of the rotorcraft in yaw.
The rotor(s) of a rotorcraft is/are conventionally driven in rotation by a power plant comprising one or more fuel-burning engines, in particular turboshaft engines. The engine(s) is/are synchronously engaged with a mechanical power drive train leading to the rotor(s) in order to drive it/them in rotation.
In addition, the power plant is used for driving rotation of various power absorber members of the rotorcraft that are engaged with the mechanical power drive train. Such power absorber members comprise, for example: a compressor of a heating, ventilating, and/or air conditioning system, or any other member that needs to be driven in rotation.
The engine(s) is/are commonly under the control of a regulator unit that controls their operation depending on the flight attitude and on the environment outside the rotorcraft. One such regulator unit is of the type commonly referred to as a full authority digital engine control (FADEC).
Depending on rotorcraft type, the use of a regulator unit is potentially directly dependent on flight commands issued by the pilot, or else it depends on regulation orders delivered by a control unit of the rotorcraft.
One such control unit is of the type commonly referred to as automatic flight control system (AFCS) and it delivers setpoints to the regulator unit for controlling the supply of fuel to the engine(s), depending on the needs of the rotorcraft. Such setpoints commonly relate to a desired speed of rotation at which the main rotor is to be driven in rotation by the engine(s).
In this context, it is necessary to protect the engine(s) from being caused to rotate at a speed that is excessive, commonly referred to as “overspeed”.
More particularly in the event of overspeed, the engine must be stopped in order to protect the members of the rotorcraft that are driven by the engine, and in particular, the rotor(s) and the blades of its rotary wing. It is also desirable to avoid damaging the engine. For this purpose, it is common practice for operation of a rotorcraft engine to depend on a protection device for protecting its individual operation.
In the event of engine overspeed being detected, the protection device causes it to be stopped immediately. Such a protection device is typically formed by a digital control circuit placed on the logic control path(s) for enabling the regulator unit to control the operation of the engine(s). Nevertheless, a sudden stop of the engine will lead to a situation that is uncomfortable for the pilot, who then needs to intervene rapidly on the behavior of the rotorcraft.
In a single-engine rotorcraft, the engine stopping requires the pilot to perform a rapid and difficult intervention that consists in putting the main rotor into autorotation so as to avoid a severe incident.
With a multi-engine rotorcraft, the main rotor continues to be driven in rotation by at least one other engine in the event of one engine failing. Since it is improbable that all of the engines of a rotorcraft will start overspeeding simultaneously, in the event of overspeed it is known to make use of a crossed mode for stopping engines.
In crossed stop mode, the operation of an engine that is still powered is maintained whatever its speed, on condition that a special mode of operation is applied for protecting it, such as a mode of control commonly referred to as one engine inoperative (OEI) mode.
In the event of an engine failing, at least one other available engine operating in OEI mode delivers a setpoint power for a predefined duration so as to enable the rotorcraft to continue flying temporarily in spite of one of the engines being unavailable.
For information about a technological background close to the present invention, reference may be made to Documents FR 2 967 213 (Eurocopter France) and FR 2 962 165 (Turbomeca), which describe such devices for protecting an overspeeding engine.
It is found that immediately stopping an overspeeding engine can be inappropriate during certain difficult stages of flight of the rotorcraft. As mentioned above, immediate stopping of an engine puts the pilot in an uncomfortable situation of needing to react rapidly on the behavior of the rotorcraft with power resources that are diminished or non-existent.
That is why proposals have been made to restrict the full authority of the protection device for causing the engine to stop in the event of engine overspeed being detected. Proposals have been made in particular to make immediate stopping of an overspeeding engine by the protection device conditional on specific stages of flight of the rotorcraft.
For example, in Document EP 1 753 939 (Goodrich Pump & Engine Control Systems Inc.), proposals are made to take account of the altitude at which the rotorcraft is flying in order to authorize the protection device to stop an overspeeding engine. More specifically, when the rotorcraft is flying at high altitude, the protection device has full authority to prevent fuel being supplied to an overspeeding engine. However, at lower altitude when the rotorcraft is close to the ground, immediate stopping of an overspeeding engine is prevented by inhibiting the operation of the protection device.
Nevertheless, such a solution is unsatisfactory. Taking account of the altitude at which the rotorcraft is flying in order to authorize or prevent an immediate stop of an overspeeding engine is not well adapted to keeping the rotorcraft under the safest possible flying conditions. At high altitude, it may be inappropriate to allow an overspeeding engine to be stopped immediately given the overall state of the rotorcraft. At low altitude, stopping of the overspeeding engine is prevented regardless of the state of flight of the rotorcraft, and thus in itself runs the risk of putting the rotorcraft in a dangerous situation.
Proposals are also made in U.S. Pat. No. 4,619,110 (M. S. More) to restrict the full authority of the protection device to cause the engine to stop on the basis of the pilot activating a control button installed on a flight control member.
Nevertheless, such a solution is unsatisfactory insofar as the pilot, when confronted with an emergency situation and under nervous strain, will then tend always to take advantage of the possibility made available of restricting the full authority of the protection device in order to continue benefiting of optimal power capacity from engine.
It therefore appears appropriate to find reasonable criteria suitable for achieving a satisfactory compromise between the possibility of immediately stopping an overspeeding engine and keeping the engine running as long as possible in order to have the safest possible flying conditions for the rotorcraft and in order to make piloting more comfortable, with this being done by means of a solution that is reliable and easy to implement.